Gravastars, hypothetical alternatives to black holes, could end up nested inside one another like a Russian Matryoshka doll – according to new calculations that combine quantum mechanics with Einstein’s general theory of relativity. If such exotic objects exist, they could reveal their presence in gravitational-wave signals.
Black holes form by the gravitational collapse of a large star, or possibly a gas cloud, to a tiny region where gravity is so strong that not even light can escape.
In 2001 the US-based physicists Pawel Mazur and Emil Mottola showed that, in theory, another object could form from such a collapse. They did this by combining Einstein’s field equations – which describe how matter and energy affect the geometry of space–time – with quantum mechanics. Their analysis revealed that quantum fluctuations could prevent the formation of a black-hole singularity during the final stages of gravitational collapse, at least in principle. Rather, a new and bizarre type of object called a gravastar would form.
No event horizon
Gravastar is a contraction of gravitational vacuum condensate star. In some ways a gravastar is like a black hole. They both have extremely strong gravitational fields and can both emit Hawking radiation. However, a gravastar does not have a singularity at its heart, nor does it have an event horizon beyond which light, matter and information can pass but never return.
Instead, a gravastar is a bubble of de Sitter space, which is a mathematical description of space filled with negative energy. As such, it provides a simple model that is consistent with an expanding universe driven by dark energy. In the conventional gravastar model this bubble of de Sitter space is initially created by the quantum fluctuations and bounded by an infinitesimally thin shell of matter.
“A de Sitter space–time wants to expand but in a gravastar it is surrounded by a shell of matter that instead wants to collapse,” says Luciano Rezolla, who is the chair of theoretical astrophysics at the Goethe University of Frankfurt. “Balancing the two opposite behaviours leads to a stable gravastar.”
Nested gravastars
Now, Rezolla’s graduate student Daniel Jampolski has found a new solution to the field equations that describes how two or more gravastars can be nested inside one another like a cosmic Matryoshka doll.
Jampolski and Rezolla call such a phenomenon a nestar, which is short for nested star. The interior structure of a nestar would feature a bubble of de Sitter space, surrounded by a shell of matter, which is then surrounded by another volume of de Sitter space that is encased by another shell of matter, and so on. In addition, rather than being infinitesimally thin, the matter shells could have a substantial thickness, in some cases making up practically the entire radius of the nestar.
“There are some nestar configurations that are given by an infinitesimally small de Sitter interior – just a point – followed by a matter interior that essentially fills the whole nestar, and then there are two thin shells near the surface, one made of de Sitter space–time, the other one of matter,” Rezzolla tells Physics World. “Because in this case the nestar would be mostly made of matter, its formation may be less exotic than in the case of a complete de Sitter interior.”
However, gravastars remain hypothetical with no observational evidence that they exist, which should lead to some caution says Paolo Pani, a professor of theoretical physics at Sapienza University of Rome, who was not involved in the study.
“A fundamental question is how such solutions – ordinary or nested gravastars – can be formed dynamically in the first place, since we do not currently have a consistent model,” Pani says.
Ringing like a bell
However, not knowing how gravastars form does not exclude their existence. Indeed, they could exist in compact binary systems that merge and produce gravitational waves.
As two compact massive objects (such as black holes or neutron stars) spiral into one another they broadcast a distinctive gravitational-wave signal called a chirp. When the objects merge to create a black hole, the gravitational waves that are emitted resemble the fading ringing of a struck bell. Both the chirp and ringdown from such mergers have been observed by the LIGO–Virgo–KAGRA gravitational wave detectors.
Such a merger could also create a gravastar or nestar, and Jampolski and Rezolla say that these would have distinctive ringdown signals. Rezolla adds, “A nestar would ringdown differently from a gravastar of the same mass because of its internal structure.” Specifically, the various shells where matter and de Sitter space interface would oscillate in a particular manner, distinct from a regular gravastar.
With 90 gravitational-wave events having been detected thus far, and another observing run currently under way, there’s been plenty of data in which to search for a gravastar signature.
“All gravitational-wave observations so far are consistent with the hypothesis that the objects are black holes or neutron stars,” says Pani. “However, the ringdown is hard to measure accurately,” he adds, which leaves some room for uncertainty.
Heating the shell
Another way in which a gravastar could reveal itself is by the accretion of matter onto its surface. In the case of a black hole, matter and light disappear beyond the event horizon, which is what the Event Horizon Telescope saw when it imaged the “shadows” of the supermassive black holes at the centre of the M87 and Milky Way galaxies. Gravastars are different in that they are horizonless. While some matter could pass through the outer shell to be absorbed by the de Sitter space–time within, more matter could impact the surface shell, making it thicker and causing it to heat up and emit light. If the Event Horizon Telescope were to ever image an actively accreting gravastar it would see this emission, albeit highly redshifted by gravity.
LIGO gravitational-wave signal backs up Hawking’s area theorem
Rezzolla admits that while the mathematics might work, a physical model describing how gravastars and nestars could exist in reality still eludes us.
“We really do not have a good idea about how gravastars form [and] since we know so little about the matter constituting gravastars, these assumptions are difficult to test,” Rezzolla says.
Jampolski and Rezzolla describe their new solution to Einstein’s field equations in the journal Classical and Quantum Gravity.
